The present invention relates to layered electrophotographic photoconductors; i.e., photoconductors having a metal ground plane member on which a charge generation layer (CGL) and a charge transport layer (CTL) are coated, in that order. Such a photoconductor may optionally include a barrier layer located between the metal layer and the CGL, and/or an adhesion promoting subbing layer located intermediate the barrier layer and the CGL, and/or an overcoat layer on the top surface of the CTL. In photoconductors of this type, the charge generation function and the charge transport function are provided by different discrete layers that are coated at different times during manufacture of the photoconductor.
As used herein, the terms dark charge and charge voltage are intended to mean the state of a photoconductor area that has been charged, but has not been exposed to light. The terms light charge and discharge voltage are intended to mean the state of a photoconductor area that has been charged and has been subsequently exposed to light.
In use, such a photoconductor is maintained in the dark as it is charged at the charging station of a reproduction device. The photoconductor must accept a high magnitude electrical charge on its outer surface at this charging station; i.e., by the use of a charge corona. This initial charge, which is defined as the dark charge, must not appreciably decrease in magnitude with time (i.e., dark decay), and the high magnitude of this initial charge must be repeatable with extended photoconductor usage or cycling.
The photoconductor's dark charge is reduced to what is defined as the white charge by image-forming radiation that strikes the photoconductor at the imaging station of the reproduction device. This white charge must be of a relatively low magnitude, and this low magnitude also must be repeatable with extended photoconductor usage or cycling.
During use in a reproduction device, the photoconductor is usually subjected to an erase source of radiation one time during each reproduction cycle. This radiation is usually of a spectral nature that can be characterized as yellow light. In addition, initial manufacture of the photoconductor and/or the reproduction device, and later servicing of the reproduction device and the like, periodically causes the photoconductor to be exposed to ambient room light. Room light can be characterized a white light, composed primarily of yellow and blue light. Room light is sometimes called blue light herein.
An acceptable photoconductor must maintain its electrical characteristics, such as dark charge and white charge, for example, after being exposed to these yellow and blue light sources (i.e., the photoconductor must not appreciably fatigue as a result of these radiation sources).
The CGL of such photoconductors usually comprises a polymeric binder containing a charge generation molecule, whereas the overlying CTL comprises a polymeric binder containing a charge transport molecule.
The charge generation molecules within the CGL are sensitive to image-forming radiation, and photo generate free electron-hole pairs within the CGL as a result of such radiation. If the surface of the photoconductor has been charged negatively, the electrons of the electron-hole pairs migrate to the photoconductor's ground plane as the holes migrate through the CTL to the charged surface of the photoconductor. In this way, the surface of the photoconductor is discharged to the white charge level, and a latent image is formed on the surface of the photoconductor.
The CTL is usually non-absorbent of the image-forming radiation, and its charge transport molecules serve the purpose of transporting holes to the surface of the photoconductor to neutralize the charge on this surface.
One of the parameters limiting the performance of a photoconductor is the charge carrier mobility of the CTL. High mobility of radiation generated charge carriers is desirable to thereby provide substantial discharge of the photoconductor in its white voltage or charge area, and the magnitude of this low charge state should not vary appreciably with photoconductor usage.
The photoconductor in its non-radiated areas must maintain its initial dark charge or voltage, and the magnitude of this high charge state should not vary appreciably with photoconductor usage.
In addition, since the CTL is usually exposed to mechanical wear (i.e., a developer station, a transfer station, and the like) the polymer of the CTL is desirably a wear-resistant polymer.
Prior art electrophotographic photoconductors have used polyester polymers and polycarbonate polymers as binders for the CTL and/or the CGL of the photoconductor. The prior art has also used mixtures of polycarbonate and polyester as binders for these layers, as well as copolymers thereof.
We have discovered that the use of an ordered copolyestercarbonate polymer (whose molecular structure is represented in FIG. 4) as the binder of the photoconductor's CGL and/or CTL provides a superior photoconductor. The ordered copolyestercarbonate polymer in accordance with the invention must have a critical range of ester content, as will be described. As a result, a photoconductor in accordance with the invention has electrical characteristics that do not appreciably change upon being subjected to yellow and/or blue light, and the photoconductor possesses good mechanical wear properties.
U.S. Pat. No. 4,330,662, which is incorporated herein by reference, describes an ordered copolyestercarbonate of the type used as a binder in accordance with the present invention. The polymeric material described in this patent is said to have superior heat resistance, clarity and impact strength, and is said to be useful for making tough transparent films.
As used herein, the term ordered copolyestercarbonate is intended to mean the material described in U.S. Pat. No. 4,330,662 and sold by the Dow Chemical Company as the brand Calibre (XP 734). The ordered molecular structure of this material is represented in FIG. 4.
An ordered copolyestercarbonate material of the type described in above-mentioned U.S. Pat. No. 4,330,662, and having a critical range of ester content in accordance with the present invention, when used as the binder for the CTL and/or the CGL of a layer photoconductor, provides a superior electrophotographic photoconductor, as will be described.
U.S. Pat. No. 4,621,038 is of interest in that it describes a layered photoconductor, and suggests that the CGL include fluorinated squaraine compounds as the photo generation molecule, and lists a number of binder polymers that may be used in the photoconductor layers, including polycarbonates, polyesters, as well as block random or alternating copolymers thereof.
U.S. Pat. No. 4,612,271 is of interest in that it teaches a photosensitive composition comprising azo compounds that can be used in optical disks and as an electrophotographic photosensitive material and suggests that, as a binder, a high molecular weight polymer may be used. A number of polymers are listed, including polyestercarbonate.
The charge generation molecule of the present invention is squarylium and preferably 2,4-bis-(4-dimethylamino-2-hyroxyphenyl) cyclobutene diylium-1,3-diolate (hereinafter referred to as OHSQ).
In this regard, U.S. Pat. No. 3,824,099, incorporated herein by reference, is of interest in that example 11 of the patent discloses the OHSQ molecule used in the present invention. This patent states that polyesters are suitable binders. The use of polycarbonate is also described.
The charge transport molecule of the present invention is a hydrazone and preferably 4-diethylaminobenzaldehyde-1,1-diphenylhydrazone (DEH).
In this regard, U.S. Pat. No. 4,150,987, incorporated herein by reference, is of interest in that this patent describes the use of a hydrazone in the CTL of a photoconductor. This patent suggests that a mixture of polyester resin and polycarbonate resin be used as a binder for the CTL.
This later patent is also of interest in that it describes the use of tetrahyrdofuran (THF) and toluene solvents to form the coating ink, these solvents also being preferred solvents of the present invention.
The prior art of which the present inventors are aware does not teach the invention's critical range of ester content for an ordered copolyestercarbonate CGL and/or CTL binder, nor the new and unusual results that flow from the use of the critical range(s) of the present invention.